1. Field of the Invention
This invention relates to a catalyst for purifying exhaust gases which is used in the exhaust system of automotive engines, and more particularly to a catalyst for purifying exhaust gases which is excellent in strength such as thermal shock resistance.
2. Description of the Related Art
Recently, global warming due to carbon dioxide has become an issue and reduction of carbon dioxide emissions has become a demanding task. In the field of automobiles, a decrease in the carbon dioxide content in exhaust gases has also been demanded, and lean-burn engines which burn lean fuel under oxygen-excessive atmospheres have been developed. Owing to their superior fuel economy, these lean-burn engines can suppress carbon dioxide emissions.
A system for these lean burn engines has been developed and reduced into practical use in which an air/fuel mixture is generally burned under fuel-lean conditions i.e., in oxygen-excessive atmospheres but intermittently burned under stoichiometric to fuel-rich conditions in order to make the exhaust gases reducing atmospheres and reduce NOx into innocuous entities. As a most suitable catalyst for this system, a NOx storage-and-reduction catalyst using a NOx storage component has been developed, which sorbs NOx in fuel-lean atmospheres and releases the sorbed NOx in stoichiometric to fuel-rich atmospheres.
For example, Japanese Unexamined Patent Publication (KOKAI) No. 5-317,652 proposed a catalyst for purifying exhaust gases in which Ba or other alkaline earth metals and Pt are loaded on a porous oxide such as γ-Al2O3. Japanese Unexamined Patent Publication (KOKAI) No. 6-031,139 proposed a catalyst for purifying exhaust gases in which K or other alkali metals and Pt are loaded on a porous oxide such as γ-Al2O3. Japanese Unexamined Patent Publication (KOKAI) No.5-168,860 proposed a catalyst for purifying exhaust gases in which La or other rare-earth elements and Pt are loaded on a porous oxide such as γ-Al2O3.
In using these NOx storage-and-reduction catalysts, exhaust gases are pulsatingly changed from fuel-lean atmospheres to stoichiometric to fuel-rich atmospheres by controlling the air/fuel ratio pulsatingly from a fuel-lean side to a stoichiometric to fuel-rich side. Hence, NOx are sorbed by a NOx storage component on the fuel-lean side and the sorbed NOx are released on the stoichiometric to fuel-rich side and reacted with and purified by reducing components such as hydrocarbons (HC) and carbon monoxide (CO) contained in large amounts in the exhaust gases. Thus, NOx in exhaust gases even from lean-burn engines can be purified at a high efficiency. HC and CO in exhaust gases can also be efficiently purified, since the HC and CO are not only oxidized by a noble metal and but also used for reducing NOx.
By the way, a catalyst for purifying exhaust gases from an automotive engine must be stably held in an exhaust gas passage of an automobile in order to increase the area of contact between exhaust gases and a catalyst ingredient while an increase in pressure loss must be suppressed as much as possible. To meet these demands, a honeycomb-shaped support substrate formed of a MgO—Al2O3—SiO2 composite oxide such as cordierite has been conventionally used and a monolithic catalyst has often been used which comprises this support substrate and a catalyst layer formed on the surface of the support substrate by loading a catalytic ingredient on a porous oxide.
However, it has been turned out that a NOx storage-and-reduction monolithic catalyst, using an alkali metal as a NOx storage component, suffers from degradation of a support substrate during its use, and deteriorates in strength such as thermal shock resistance when compared with other catalysts such as three-way catalysts. For example, in the case of a NOx storage-and-reduction catalyst for purifying exhaust gases employing, as a substrate, a structural member such as cordierite composed of a MgO—Al2O3—SiO2 composite oxide and having a catalyst layer formed by loading a noble metal and an alkali metal on a porous oxide such as alumina, the thermal expansion coefficient is larger than that of a catalyst which does not include any alkali metal. As the thermal expansion coefficient is larger, the thermal shock resistance is lower and the strength is also lower.
Investigation on distribution of the alkali metal loaded in a catalyst which got a large thermal expansion coefficient has showed the existence of the alkali metal even in the substrate. Consequently, it has been clarified that the cause of strength deterioration lies in the fact that the alkali metal reacts with a component (SiO2 in particular) of the substrate in the boundary of the catalyst layer and the substrate, which changes the composite oxide composition of the substrate.
In this connection, Japanese Unexamined Patent Publication (KOKAI) No.2000-279810 has proposed a catalyst for purifying exhaust gases which has a zirconia layer between a catalyst layer loaded with a noble metal and a NOx storage component, and a substrate. Since zirconia hardly reacts with alkali metals, the zirconia layer inhibits an alkali metal from moving into the substrate. A decrease in strength of the substrate can be thus suppressed.
However, the catalyst for purifying exhaust gases disclosed in Japanese Unexamined Patent Publication (KOKAI) No.2000-279,810 still suffers from a problem in that the zirconia layer inevitably has pores between zirconia particles, and the alkali metal passes through these pores and reaches and reacts with the substrate, which results in a decrease in strength of the substrate.